1. Field of the Invention
The present invention relates to a battery pack using nonaqueous electrolyte batteries and to a vehicle using the battery pack.
2. Description of the Related Art
A nonaqueous electrolyte battery has attracted attentions as a battery having a high energy density, and vigorous research is being conducted on a nonaqueous electrolyte battery that is charged and discharged by the migration of Li ions between the negative electrode and the positive electrode.
Various properties are required for the nonaqueous electrolyte battery depending on the use of the battery. For example, long charge-discharge cycle characteristics under a high temperature environment are required when it comes to a nonaqueous electrolyte battery mounted on a vehicle such as a hybrid electric automobile or used for the emergency power supply of an electronic apparatus.
Generally used nowadays is a nonaqueous electrolyte battery in which a lithium-transition metal composite oxide is used as a positive electrode active material and a carbonaceous material is used as a negative electrode active material.
In recent years, a nonaqueous electrolyte battery in which a lithium-titanium composite oxide having a Li absorption-release potential higher than 1.0 V is used as the negative electrode active material has been put to the practical use. Since the lithium-titanium composite oxide is small in the change of volume accompanying the charge-discharge operation of the battery, the lithium-titanium composite oxide is expected to impart excellent charge-discharge cycle characteristics to the battery.
Among the lithium-titanium composite oxides, the spinel type lithium-titanium composite oxide is particularly excellent in the charge-discharge cycle characteristics and is expected to be prominently useful. Japanese Patent Disclosure (Kokai) No. 9-199179 discloses a lithium ion battery in which lithium titanate is used as a negative electrode material. The lithium-titanium composite oxide also includes ramsdellite type lithium-titanium composite oxide having a composition formula of Li2Ti3O7, as disclosed in Japanese Patent Disclosure No. 2000-12090.
On the other hand, Japanese Patent Disclosure No. 10-69922 discloses a nonaqueous electrolyte lithium secondary battery comprising a negative electrode formed mainly of the spinel type lithium-titanium composite oxide and a positive electrode having a potential higher than that of the spinel type lithium-titanium composite oxide. It is disclosed in this patent document that, if the ratio of the electric capacity of the negative electrode to that of the positive electrode is increased to 1.1 or 1.2, the positive electrode potential is markedly lowered to a value in the vicinity of zero in the last period of the discharge to degrade the positive electrode characteristics, leading to deterioration of the capacity.
On the other hand, Japanese Patent Disclosure No. 2004-171955 relates to a bipolar battery comprising a bipolar electrode including a current collector, a positive electrode active material layer formed on one surface of the current collector, and a negative electrode active material layer formed on the other surface of the current collector. It is disclosed that a plurality of bipolar electrodes constructed as described above are stacked one upon the other with an electrolyte layer sandwiched therebetween so as to provide the bipolar battery. In this prior art, a changeable electrode active material and an unchangeable electrode active material are used in the electrode active material layers. The changeable electrode active material exhibits the characteristics that, when the charge capacity is reached, the voltage is changed in a manner to exceed the rate of change of the voltage before the charge capacity is reached. On the other hand, unchangeable electrode active material exhibits the characteristics that, even if the charge capacity of the changeable electrode active material is reached, the rate of change of the voltage is substantially left unchanged. In this prior art, the changeable electrode active material is used in the positive electrode active material layer or the negative electrode active material layer, and the unchangeable electrode material is used in the other electrode active material layer. The particular construction permits the bipolar battery to exhibit the characteristics that the change of voltage in the end period of the charging is increased in a manner to exceed the previous rate of change of the voltage. As a result, it is possible to prevent an over charge of the battery module. In Example 1 of Japanese Patent Disclosure No. 2004-171955 referred to above, the positive electrode active material layer containing spinel type lithium manganate is used as the changeable electrode, and a negative electrode active material layer containing lithium titanate is used as the unchangeable electrode. It is taught that the charge capacity of the negative electrode active material layer (unchangeable electrode) is set at 120% of the charge capacity of the positive electrode active material layer (changeable electrode) so as to increase the change of voltage in the end period of the charging of the bipolar battery. It is also taught in Example 2 of the patent document quoted above that, if the charge capacity of the positive electrode active material layer is set at 120% of the charge capacity of the negative electrode active material layer, it is possible to change the negative electrode active material layer into the changeable electrode. Further, it is taught in this patent document that LiFePO4 can be used as the positive electrode active material.